1. Technical Field
The embodiments herein generally relate to electrical circuits, and, more particularly, to techniques for implementing an analog variable gain amplifier (VGA) to enhance overall system signal processing.
2. Description of the Related Art
To realize a dB-linear VGA circuit in a non-bipolar junction transistor (BJT) technology, the exponential automatic gain control (AGC) characteristics is approximated by Equation (1): ex=(1+x)/(1−x). One method to realize the VGA is shown in FIG. 1. The AGC control voltage is applied to an exponential converter circuit that converts the AGC control voltage according to the previous expression (or other approximations). The output voltage of this circuit is then fed into a control port of a multiplier. The signal is then applied to the input port of the multiplier circuit. The multiplier circuit will linearly multiply the input signal with the signal applied to the control port. Hence, the dB-Linear VGA is realized.
Another method to realize the VGA is shown in FIG. 2. The circuit shows two cascaded transconductor stages. The gain of such a configuration is the ratio of transconductances, Equation (2), A=gm1/gm2. If an electronically tuned gm stage is used, then the dB-linear gain can be realized by increasing the tuning signal of gm1 (the controlling signal can be a voltage Vc1 or a biasing current Ib1) while simultaneously reducing the gm2 tuning signal (can be a voltage Vc2 or a current Ib2). This directly implements a gain that changes exponentially with the AGC control voltage.
Unfortunately, the methods of FIGS. 1 and 2 generally suffer from, first, limited signal headroom. This is mainly caused by the difficulty of realizing multipliers or tunable gm stages with rail-to-rail swing. Furthermore, as integrated circuits (ICs) continue to migrate towards deep sub-micron technologies, the allowed supply voltage is becoming more limited and maintaining wider signal headroom is seen as being crucial for maintaining good dynamic range and lower power consumption.
Second, the methods of FIGS. 1 and 2 tend to suffer from noise performance. In the first method (FIG. 1), the noise of the exponential converter circuit is amplitude modulated by the input signal. Hence, the exponential converter circuit has a significant contribution to the total noise of the VGA. Furthermore, the realization of a linear multiplier circuit involves the use of many active devices in the signal path. The noise and non-ideality of those devices generally leads to poor noise performance (as compared to op-amp based circuits).
Third, the methods of FIGS. 1 and 2 tend to suffer from limited VGA linearity. This is because realizing linear multipliers or tunable gm-transconductors is achieved through circuit techniques that linearize the characteristics of the active devices used. Such techniques are sensitive to device non-idealities as well component mismatches. Generally, this results in signal distortion that limits the overall dynamic range of the VGA.
Fourth, the methods of FIGS. 1 and 2 tend to suffer from VGA gain mismatches. Both conventional methods rely on transistors' tranconductance in achieving the VGA function. Hence, in applications where the use of matched VGAs is required, VGA circuits realized using these conventional methods will generally be harder to use (statistically, a larger percentage of components will exhibit non-acceptable mismatches).
Fifth, with respect to the methods of FIGS. 1 and 2, in most cases, the output of the transconductor as well as the multiplier is un-buffered. Hence, the VGA typically cannot drive a load resistance directly. Accordingly, an extra buffer stage is necessary to achieve this.
In conclusion, the conventional methods, as illustrated in FIGS. 1 and 2, offer clear disadvantages relating to: (1) limited signal swing; (2) higher noise that limits the minimum signal that the VGA circuits can process; (3) degraded over-all linearity that results in signal distortion, especially at higher signal levels; (4) difficulty in achieving sufficiently good matching between similar VGA circuits; and (5) in most cases, the outputs of the transconductor as well as the multiplier is un-buffered. Hence, the VGA cannot drive a load resistance directly, whereby an extra buffer stage is necessary to achieve this. In view of the drawbacks and limitations of the conventional techniques, there remains a need for a new technique for realizing dB-linear VGAs.